1. Field of the Invention
The present invention relates to a driver circuit for an audible indicator and in particular for a buzzer controlled by a transistor.
2. Discussion of Related Art
Buzzers are used for a wide variety of indicators including toys, cellular telephones, pagers and other communication devices and other miscellaneous and asundry devices which require an audible indicator. The buzzers of course require driver circuits.
One such prior art driver circuit it shown in FIG. 1. FIG. 1 consists of a driver circuit 10 which includes a buzzer 11 a bipolar or field effect transistor 12. The buzzer 11 and the transistor 12 are connected in series between a voltage potential V.sub.DD of, e.g., 6.2 volts, and ground. Connected in parallel to the buzzer 11 are two parallel connected diodes 13 and 14. The cathodes of the diodes 13 and 14 are connected to the voltage source V.sub.DD and the anodes of the diodes 13 and 14 are connected to the node 1 interconnecting the buzzer 11 and the transistor 12.
The function of the buzzer driver 10 is to charge the buzzer 11, which works as an inductor element, when the transistor 12 is conducting. When the transistor 12 does not conduct, the inductive element of the buzzer 11 discharges through the diodes 13 and 14. The sound is created by vibration of a membrane on top of the inductor due to this driving. Depending on what frequency is being input to the base or gate of the transistor, a certain tone is produced. The level of sound is determined by the width of the pulses supplied to the gate (or base) of the transistor 12 via a pulse with modulation (PWM) circuit (not shown), as well as the level of the supply voltage V.sub.DD.
Because the diodes 13 and 14 do not have a significant reverse-bias potential, they develop only approximately 0.6 volts across the buzzer. Unnecessary losses result due to the use of these conventional buzzer diodes. These loses mean that the life of a battery (in a battery operated circuit) is shorter than necessary.
Further, as a consequence of the buzzer potential being reduced is that the level of the output sound is reduced. FIG. 1a illustrates the buzzer voltage curve 15 and current curve 16 of the conventional buzzer driver shown in FIG. 1.